1. Field of the Invention
The present invention relates to a technique for suppressing the bowing of an epitaxial wafer prepared by forming a group III-nitride multilayer film on a substrate.
2. Description of the Background Art
Group III-nitride semiconductors such as GaN-based compound semiconductors have a wide band gap, a high breakdown voltage and high saturation electron velocity compared with other semiconductors. Therefore, the group III-nitride semiconductors have frequently been studied as the material for optical devices such as light-emitting diodes, laser diodes (LDs) and photodetectors, and for electronic devices such as high-electron-mobility transistors (HEMTs) and hetero bipolar transistors (HBTs).
It is difficult to grow large-size bulk single-crystal GaN-based compound semiconductors. Therefore, sapphire or SiC substrates commonly are used for the growth of the GaN-based material by metal-organic chemical vapor deposition (MOCVD). However, a mismatch of the lattice constants between the GaN-based compound semiconductor and the substrates is so large that a high-quality epitaxial layer can not be obtained by directly forming the target GaN-based compound semiconductor on the substrate. Therefore, a technique for forming a buffer layer of AlN or GaN has been widely employed. The buffer layers are formed on the substrate in advance of formation of the epitaxial layer of the GaN-based compound semiconductor. For example, Japanese Patent Application Laid-Open Gazette No. 8-8217 (1998) discloses a technique of forming a buffer layer consisting of AlX1Ga1-X1N (0≦X1≦1) prepared at a temperature growing no high-quality single crystal between a target epitaxial layer and a substrate.
FIG. 14 is a schematic view of an epitaxial wafer prepared by using this technique of forming a buffer layer. As understood from FIG. 14, an epitaxial wafer 5 is prepared by epitaxially growing a target group III-nitride layer 53 on a prescribed substrate 51 with a buffer layer 52. For example, an (AlxGay)N (0≦x≦1, 0≦y≦1, x+y=1) layer (low-temperature-deposited buffer layer having low crystallinity) of 20 nm to 50 nm in thickness formed by MOCVD at a temperature of 400° C. to 600° C. or an AlN layer (high-temperature-grown buffer layer having high crystallinity) of about 1 μm in thickness formed by MOCVD at a temperature of at least 1000° C. or more can be applied for the buffer layer 52. A target group III-nitride layer 53 is epitaxially formed on the buffer layer 52 by MOCVD at a temperature of 1000° C. to 1300° C. Thus, a high-quality epitaxial single crystalline layer can be formed on a substrate by forming a target group III-nitride layer with a buffer layer.
According to the conventional art, however, the epitaxial wafer provided with the group III-nitride layer on its surface may experience bowing due to the difference of the thermal expansion coefficients of between the group III-nitride layer and the substrate. This bowing disturbs the vacuum chuck properties during transfer steps or reduces the exposure accuracy in a photolithography step. Further, this bowing may cause cracking on the epitaxial wafer. Therefore, a technique for suppressing the bowing of the epitaxial wafer is strongly demanded.